Various data storage systems have been developed and deployed in recent years, which include those known as “storage area network” (SAN). A SAN is a system of storage devices such as hard disk drives (HDD) and magnetic tape drives made accessible to servers or other computers via a network. Data migration may take place in a SAN to move data from existing storage devices to new storage devices with a larger data capacity. The migration of data could be a time-consuming process because of the massive size of data that needs to be moved.
One method of data migration uses a server for virtualizing the volumes that may be accessed by host computers, so that the migration of data is transparent to those hosts. This method causes the server to run a background process to copy data from a first disk (physical space of virtual volumes) to a second disk, concurrently with its regular work of controlling access requests from hosts to virtual volumes. When the copy of data is completed, the server switches the physical space of virtual volumes, from the first disk to the second disk.
According to another method of data migration, the current storage device and a new storage device are first registered as members of a virtual unification device, and all files in the current storage device are migrated to the new storage device. The method then deregisters the current storage device from the virtual unification device upon completion of the migration.
See, for example, the following documents:                Japanese Laid-open Patent Publication No. 2001-331355        Japanese Laid-open Patent Publication No. 2004-046661        
The virtualization of an existing real storage device results in a plurality of paths for a server to reach the real storage device, since new paths are produced for virtual storage devices, in addition to existing paths for access to the real storage device. Here the concurrent use of an existing access path and an added access path is inhibited to avoid possible data corruption in the real storage space. This means, however, that there exists a certain time period during which the storage device is inaccessible.